1. Techinical Field
This invention relates to printers for printing on a moving web and, more particularly, to a method and associated apparatus for adjusting the lateral position of the printhead over the labels in label printers having a printhead mounted above a path for printing on labels carried by a backing strip along the path comprising the steps of, mounting the printhead over the path for adjustable lateral movement; moving a backing strip with labels thereon along the path until lateral stability is established; and, adjusting the lateral position of the printhead over the labels to a desired position with respect to the labels.
2. Background Art
In a label printer such as that generally indicated as 10 in FIG. 1, a plurality of labels 12 are releasably attached to a backing strip 14 forming a strip of media 15 that extends from a supply roll 16 over a plurality of guide rollers 18 to a printhead 20. At the printhead 20, ink from a ribbon 22 extending between a supply roll 24 and a take-up roll 26 is transferred to the labels 12. After printing, the labels 12 are separated from the backing strip 14 by a separator 27 and the backing strip 14 is wound onto a take-up roll 28 for later disposal. The labels 12 and backing strip 14 are moved in combination from the supply roll 16 to the printhead 20 by a driven platen roller roller 30 which also supports the labels 12 and backing strip 14 under the printhead 20 during the printing process. To keep the cost of the printer 10 low, the take-up roll 26, the take-up roll 28, and the platen roller roller 30 are all driven directly or indirectly by a single stepping motor 32 as indicated by the dashed lines. The movement of the stepping motor 32 is under the control of logic 34.
In label printing as in many areas, simplicity and cost are major factors. Consumers want the print quality they require in the least expensive printer. This is particularly true in on demand label printers. And, the introduction of small, narrow labels such as employed as labels for printed circuit boards printed in the "picket fence" mode have made the problem even more severe. Accurate lateral placement of the printing on these small labels is critical--particularly if there is any pre-printed material on them. And, at the same time, the cost of the printer is to remain low. Usually, such factors are a trade-off. That is, accurate placement can be obtained in a highly-complicated, high-cost printer. The prior art does not provide a way of obtaining both.
The problem is best understood with primary reference to FIG. 2 in combination with FIG. 1. If the guide rollers 18, platen roller 30, supply roll holding shaft 42, and the take-up roller 28 were all perfectly parallel to one another, the media 15 might be laterally positioned in the same place from one printer 10 to another. The fact is that they are not perfectly parallel. All mechanical devices are built to tolerances. And, it is a fact of manufacturing that the higher the tolerances, the more expensive the manufacturing process. Thus, if one is building a printer with low cost as an objective, the tolerances have to be lower in order to attain that objective. In addition, the lateral positioning of the labels 12 on the backing 14 are subject to tolerance variations. As a result, when the media 15 is loaded into various printers 10 of the same type, it will seek a stable lateral position under the printhead 20. If we use one edge 32 of the platen roller 20 as a lateral reference point, there will be varying space "EP" between the edge 32 and the edge 34 of the backing 16 and a varying space "EW" between the edge 34 of the backing strip 14 and the edge 36 of the labels 12. Thus, if one is required to exactly position the printhead 20 over the path of the labels 12 in order to accurately print new information in relation to pre-printed information on the labels 12, it is an impossible task using prior art techniques. This can be seen in FIG. 3 which depicts the prior art approach to mounting the printhead 20.
A shaft 38 is carried for rotation by two end bearings 39 between two sidewalls 40 of the printer 10 above and slightly behind the platen roller 30. A block 42 is fixedly mounted for rotation with the shaft 38. The lateral position of the block 42 is fixed on the shaft 38. A second shaft 50 extends outward perpendicular to the front face of the block 42. The printhead 20 is rotatably mounted on the second shaft 50. Through the use of adjusting screws (not shown), the printhead 20 can thus be adjusted to be parallel to the platen roller 30 by rotating it around the second shaft 50 and can be adjusted as to it spacing above the platen roller 30 (and, therefore, above the labels 18), by rotating the block 42 and the shaft 38 in combination. As can be seen, there is no lateral adjustability. So, the lateral positioning of the labels 12 under the printhead 20 varies from printer to printer as a function of the variations in tolerances as described above.
Wherefore, it is an object of the present invention to provide methods and apparatus for making a low-cost web printer in which the lateral positioning of the printhead is repeatable from printer to printer.
It is another object of the present invention to provide methods and apparatus for making a low-cost label printer in which the lateral positioning of the printhead on the labels is repeatable from printer to printer without regard to positional variations caused by printer and media production tolerances.
It is still another object of the present invention to provide methods and apparatus for making a low-cost label printer in which the lateral positioning of the printhead on the labels is dynamically adjustable to account for positional variations caused by printer and media production tolerances, and the like.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the drawing figures which accompany it.